One engine system being developed for controlled auto-ignition combustion operation comprises an internal combustion engine designed to operate under an Otto cycle. The engine, equipped with direct in-cylinder fuel-injection, operates in a controlled auto-ignition mode under specific engine operating conditions to achieve improved engine fuel efficiency. A spark ignition system is employed to supplement the auto-ignition combustion process during specific operating conditions. Such engines are referred to as Homogeneous Charge Compression Ignition (hereinafter ‘HCCI’) engines.
An HCCI engine operating in HCCI combustion mode creates a charge mixture of combusted gases, air, and fuel in a combustion chamber, and auto-ignition is initiated simultaneously from many ignition sites within the charge mixture during a compression stroke, resulting in stable power output, high thermal efficiency and low emissions. The combustion is highly diluted and uniformly distributed throughout the charge mixture, resulting in low burnt gas temperature and NOx emissions typically substantially lower than NOx emissions of either a traditional spark ignition engine, or a traditional diesel engine.
HCCI has been demonstrated in two-stroke gasoline engines using conventional compression ratios. It is believed that the high proportion of burnt gases remaining from the previous cycle, i.e., the residual content, within the two-stroke engine combustion chamber is responsible for providing the high mixture temperature necessary to promote auto-ignition in a highly diluted mixture.
In four-stroke engines with traditional valve means, the residual content is low and HCCI at part load is difficult to achieve. Known methods to induce HCCI at low and part loads include: 1) intake air heating, 2) variable compression ratio, and 3) blending gasoline with ignition promoters to create a more easily ignitable mixture than gasoline. In all the above methods, the range of engine speeds and loads in which HCCI can be achieved is relatively narrow. Extended range HCCI has been demonstrated in four-stroke gasoline engines using variable valve actuation with certain valve control strategies that effect a high proportion of residual combustion products from previous combustion cycle necessary for HCCI in a highly diluted mixture. With such valve strategies, the range of engine speeds and loads in which HCCI can be achieved is greatly expanded using a conventional compression ratio.
However, even with such valve control strategies, high load HCCI engine operation is limited by NOx emissions and by in-cylinder pressure rise or amplitude of pressure oscillation at high load. Too large a pressure rise or amplitude of pressure oscillation results in unacceptable combustion generated noise (i.e. knocking or ringing). And, since there is a relatively strong correlation between NOx emission and knock/ringing, either parameter may provide a capable indicator of a high load operating limit. Beyond an acceptable high load limit, the valve control strategy limits residual combustion products to effect in cylinder conditions which will not produce auto-ignition. At loads beyond the high load limit for HCCI combustion, the valve phasing is controlled at low lift to enable unthrottled spark-ignition (hereafter ‘SI’) combustion control in order to effect desired combustion phasing. An upper load limit and corresponding maximum torque output is reached with unthrottled SI combustion. Achieving engine output beyond the unthrottled SI combustion limit requires full valve lift operation and intake air throttling which increases engine pumping losses.
The HCCI combustion process is strongly influenced by combustion chamber surface temperature as seen, for example, in the correlation of engine coolant temperature or level of combustion chamber deposits to HCCI engine combustion phasing. More particularly, combustion phasing is known to advance with increasing coolant temperature or thickness of combustion chamber deposits. Although active coolant temperature control can be used to extend high load operation in both HCCI and SI modes of HCCI engines, the inherently long response time of engine thermal management systems does not enable rapid control adjustments necessary for dynamic cycle-to-cycle engine control.